As is well known, a communication system may utilize multiple communication channels to communicate signals between transmitters and receivers of the system. For example, multiple channels may be used to separate different transmitted data signals from one another, or to provide an increased data rate.
A problem that can arise in multiple channel communication systems relates to crosstalk between the various channels, also referred to as inter-channel crosstalk. For example, digital subscriber line (DSL) broadband access systems typically employ discrete multi-tone (DMT) modulation over twisted-pair copper wires. One of the major impairments in such systems is crosstalk between multiple subscriber lines within the same binder or across binders. Thus, a transmission on one subscriber line may be detected on other subscriber lines, leading to interference that can degrade the throughput performance of the system. More generally, a given “victim” channel may experience crosstalk from multiple “disturber” channels, again leading to undesirable interference.
Dynamic spectrum management (DSM) techniques have been developed in order to combat interference and to maximize the effective throughput and reach of a DSL system. Lower level DSM techniques, commonly referred to as Level 1 and Level 2 techniques, generally adjust the power levels associated with a given subscriber line in an attempt to minimize interference. Level 3 techniques are more sophisticated, and allow active cancellation of inter-channel crosstalk through the use of a precoder. The precoder is typically used to achieve crosstalk cancellation for downstream communications between a central office (CO) and customer premises equipment (CPE). It is also possible to implement crosstalk control for upstream communications from the CPE to the CO, using so-called post-compensation techniques.
One known approach to estimating crosstalk coefficients for downstream power control or crosstalk cancellation in a DSL system involves transmitting distinct pilot signals over respective subscriber lines between a CO and respective CPE of the system. Error feedback from the CPE based on the transmitted pilot signals is then used to estimate crosstalk. Other known approaches involve perturbation of precoder coefficients and feedback of signal-to-noise ratio (SNR) or other interference information.
Crosstalk estimates are commonly utilized in situations in which it is necessary to “join” an additional line to a group of active lines in a DSL system. For example, it may become necessary to activate one or more inactive lines in a synchronization group that already includes multiple active lines. Such joining of an additional line may require that the power control or precoder be adjusted accordingly in order to optimize system performance.
Crosstalk estimates are also used in a variety of other line management applications, including determining whether or not to precode, and setting power levels on victim and disturber lines.
Accordingly, it is important to have accurate crosstalk estimates that can be generated quickly and efficiently.